Radio transmitters usually have a fixed transmit power level, although some can be manually adjusted to reduce the power level of a transmitted signal below the maximum rated output power of the unit. For long distance point-to-point communications, transmissions are normally made at the full rated power of the transmitter, thereby insuring the maximum likelihood of success in communicating with the intended recipient. However, for short range communications, it is often desirable to moderate the transmission power level, for example, to avoid overloading the front end of a nearby receiver, or to minimize the area over which the radio signals can be received and thus, the possibility of causing interference with other more distant transmitters that are using the same frequency.
In a network of stations communicating with each other, the reception between any two member stations can vary from time to time, particularly if the stations are mounted on mobile platforms so that the distance and propagation path between the stations continually change. For the operator of a member station in such a network, it can be relatively difficult to optimally manually adjust the transmitter power level to maintain communications with all other member stations. The transmit power level used by a given station should be just sufficient to establish and maintain effective communications with the station in the network that has the poorest reception quality. Typically, an operator attempting to communicate with selected members of a network will manually increase the transmit power level of his transmitter until each station with which he is attempting to communicate reports back that they have heard his transmission. This process can require several minutes and likely must be repeated after only a few minutes have elapsed if signal propagation conditions between member stations are changing rapidly. Clearly, it would be beneficial to have an automated system that adjusts the transmit power level without human intervention.
U.S. Pat. No. 3,732,496 discloses a radio transmitter and receiver that include means for automatically adjusting the transmission power level. In this patent, an attenuator providing a stepped variable attenuation level is controlled by the frequency of a signal received from another transmitter/receiver in the network. Each station in the network includes means for transmitting a signal at a specific frequency selected from a predetermined set of such frequencies; the frequency selected is indicative of the level at which a transmission from another station in the network is received. A selector device at each station includes a set of threshold circuits that determine the relative signal strength of a received signal and select appropriate combinations of frequencies for the signal transmitted back to the original transmitter. This signal indicates the reduction (in dB) that the original transmitter should apply to its transmit power to achieve a desired power level.
A slightly different solution to this problem is disclosed in U.S. Pat. No. 3,925,782. In this reference, transmissions by one of the member stations in a simplex network are periodically interrupted to transmit an interrogating code to the other stations asking for a power correction request from each. Power correction requests are sequentially received from the other stations during allotted time slots. The sending station then adjusts its transmit power level consistent with the lowest power reduction request that was received from the other stations.
A disadvantage of the systems disclosed in both of the above-referenced patents is that a transmitter of a station that wants to communicate with another station initially transmits a signal at a relatively high power to establish communications and then reduces its transmit power level in response to a signal from another station indicating an appropriate transmit power level reduction. As a result, an excessive transmit power level will often initially be used to establish communications between the stations. In certain instances, it may be very important to use the minimum power level required for effective communications at all times. Use of an excessive transmit power level to initiate communications between stations in a network clearly violates this requirement.
The present invention addresses the problems described above in a different manner than is disclosed in the prior art. Advantages of the present invention will become more readily appreciated as the same becomes better understood by reference to the detailed description provided below, when taken in conjunction with the accompanying drawings.